Heart failure is a common public health problem, affecting over 5 million people in the U.S. alone. The annual mortality rate due to heart failure is estimated to be 20-25%. Heart failure is a costly disease, both financially and in loss of life. Recent improvements in the surgical and medical management of heart disease have made it more important that heart disease patients are early and accurately classified into high and low risk groups. By examining indicators that are more indicative of mortality risk, high risk patients can receive more targeted treatment. One set independent mortality indicators for heart failure patients are imbalances in the autonomic nervous system.
The autonomic nervous system is responsible for maintaining a relatively constant internal physiological environment by controlling such involuntary functions as digestion, respiration, perspiration, and metabolism, and by modulating blood pressure. The autonomic nervous system is divided into two subsystems, the sympathetic and the parasympathetic. The sympathetic subsystem is responsible for providing responses and energy needed to cope with stressful situations. In response to such stress, the sympathetic system increases the level of certain autonomic activity including heart rate and blood pressure. The parasympathetic nervous system, in contrast, conserves energy by, for example, slowing the heart rate and increasing intestinal and gland activity. The parasympathetic nervous system acts to reverse the effects of the sympathetic nervous system.
Increased sympathetic and depressed parasympathetic nervous activity is common in heart failure. Autonomic imbalance is suspected of predisposing the heart to chronic ventricular dysfunction. This imbalance may be characterized a marked augmentation of sympathetic drive as well as an attenuation of parasympathetic tone. This disruption of autonomic balance appears to significantly contribute to the vasoconstriction that accompanies ventricular failure. Autonomic imbalance also predisposes the heart to ventricular arrhythmias, therefore is reported to be an independent risk factor for the mortality in heart failure population.
Measuring autonomic activity may useful for many purposes. Accurately detecting and determining the level or degree of heart failure in a patient is a particularly desirable use for such measurements. The present invention describes methods, systems, and apparatuses for assessing autonomic activity. Such assessment can be used, among other things, for early identification of patients with high risk of sudden death to enable early, aggressive intervention, and offers various other advantages over the prior art.